The invention relates generally to risers that connect offshore drilling vessels or tension leg platforms (TLPs) to blowout preventer stacks (BOPs) or production modules in deep water. More specifically, the invention relates to flotation assemblies which may be attached to the risers to counteract or offset a portion of the weight of the submerged riser pipe, maintain the riser in tension, and/or maintain the riser in a vertical position.
Risers systems are often attached to seabed systems on the ocean floor. The water depths at which the riser system is installed may be in deep water (in excess of 5,000 feet), and, currently, the trend in the industry is toward the development of drill sites in even deeper water including depths of 10,000 ft. and beyond. The riser system which must span this depth is made up of a series of structural riser pipe sections called "riser joints," generally 50 feet in length, having mechanical connections at both ends. The riser system may also include an upper riser assembly and a lower riser assembly. To prevent the riser from buckling and to compensate for the weight of the riser system it is kept in tension by the platform or vessel, or provided with buoyancy devices, often in the form of modules or shaped elements that attach to a riser.
There are several types of buoyancy modules that have been used in the industry. One particular type of buoyancy material in use in the marine drilling industry is syntactic foam, which has found extensive marine uses in applications requiring a buoyancy material capable of withstanding relatively high hydrostatic pressures. The compartmentalized structure of syntactic foam tends to localize failure as compared to single wall pressure vessels which fail catastrophically. In general, syntactic foam consists of hollow glass microspheres and epoxy binder or a high strength plastic matrix. It is especially suitable for marine applications because of its high strength and low density, allowing the foam to provide buoyancy while withstanding pressure from deep water.
In the past, semi-annular syntactic foam flotation modules have been clamped to the riser, or strapped together around the diameter of the riser joints. This type of system is "passive flotation," that is, the buoyancy of the syntactic foam modules cannot be adjusted after installation.
Syntactic foam, or similar buoyancy foams, may be manufactured in a variety of densities as required by the water depth. The trend toward even greater drilling and production capability with respect to the ultimate depth of the water at the drill-site affects the density requirement of the buoyant materials used to provide passive flotation, for example, syntactic foam modules. As the water depth increases, the buoyancy required per length of riser joint increases accordingly. This results in increased diameter and weight of individual flotation modules.
The increase in the size of these modules reduces or eliminates the ability to construct a buoyant riser for use below 12,000 feet that will run in through a standard 48" rotary table on offshore drilling vessels or on TLP drilling rigs. Because this is an industry standard size, it would, in most cases, be impossible, or at least impractical, to reconfigure a drilling vessel to accommodate a larger diameter rotary table. Moreover, the increased weight of a large diameter buoyant module results in difficulties in both handling and storage. As such, the current system of passive flotation using syntactic foam modules, with the drilling equipment in use today, may be incapable of providing the buoyancy needed to keep risers in tension at greater depths.
Another type of buoyancy system that has been used for underwater risers is an open-ended air can (canister) system. Typically, in this type of system a plurality of cans having an open bottom are attached to the riser. The cans are disposed with their bottoms open toward the seabed. A compressed air (or other gas) conduit from the surface fills the bottom-most can, displacing the water in the can. Another conduit allows the compressed air to flow into the immediately-above adjacent can, and a valve may be employed to ensure that the second and later cans are air-filled only after the air in the first can reaches a desired level of water displacement. This proceeds until all the cans are filled with air, or the desired buoyancy affect is achieved. This type of can system is an "active flotation system," in that the supply of air, and the corresponding net buoyant effect, can be controlled.
The canister system may alternatively derive buoyancy by displacement of water from the annulus between the OD of the riser casing and the ID of an outer housing (the canister) with compressed air or gas. A shut-off valve within the canister annulus controls the height of the gas/liquid level above the open end of the outer canister housing, thus trapping the gas in the canister.
As with the syntactic foam modules, air can systems must provide progressively greater buoyancy along the axis of the riser as the water depth and weight of the riser above increase; as such, progressively larger volume cans are required. At greater depths, the differential pressure between the sea-water outside the can and the compressed air inside the can is larger. The air cans may be fabricated from a number of materials (usually steel casing), most of which add to the weight and stiffness of the riser joints and may contribute additional stresses to the couplings. Moreover, the increased weight associated with the thicker walls needed at greater depths offsets a portion of the total buoyant force of the can system.
Another problem associated with current riser systems is an inability to quickly disconnect the riser from the vessel or platform when storms require the vessel or platform to move to safety; the remaining riser sections must survive the storm without losing all or part of the flotation system, while still being kept in tension and vertical.
A semi-submersible, drill ship, or TLP operating in the Gulf of Mexico or other deep water locations in the world will usually employ a "guidelineless" re-entry system because of the extreme water depth (8,000 ft. to 12,000 ft) as shown in FIG. 1. Vessels equipped with dynamically positioned automatic station keeping systems employ no mooring lines and are subject to the need to move off location during significant storms and during the hurricane season. This subjects the riser string to an "emergency disconnect" and potential catastrophic failure.
At present, when a drill ship must abandon a location due to a hurricane warning, the drilling riser (made up of joints of pipe connected by riser couplings) and the drill-string that has been deployed, must be recovered to the deck of the drilling vessel. This normally involves shearing the drill string at the subsea BOP using shear rams, unlocking the "lower marine riser package" (LMRP) from the blowout preventer stack and retrieving the drill-string and riser to the rig floor (FIG. 1). The drill pipe must be stored on the drilling vessel, followed by the riser joints, or storage provided on other vessels, which may be difficult in the extreme weather attendant with an approaching storm.
Technology is available that may significantly reduce the need for storage of long strings of drill pipe and risers during emergency disconnect. For example, the method and apparatus disclosed in U.S. Pat. No. 5,676,209 provides a subsea riser system with an upper stack of blowout preventers and an air buoyancy chamber placed in the riser at about 500 ft below the ocean surface (where lateral currents are minimal). Attached buoyancy modules below the BOP stack maintain the riser between the two BOP's generally in tension and vertical. Drill pipe may now be sheared in the upper BOP, leaving only the upper joints of drill pipe to be recovered to the rig floor during the emergency. The lower string remains in the well and/or riser until after the emergency, thus, protecting the drilling fluid in the riser and the casing.
However, with this type of technology, it is important that the flotation system associated with the riser sections that remain attached to the seabed be able to maintain the riser in tension and vertical, and that it is recoverable if necessary through the rotary table if repair and/or replacement is needed after reconnecting. The flotation apparatus of the current invention would be beneficial with the riser remaining connected to the subsea well, as well as with the riser string above the uppermost BOP stack.